Preheat the oven to 350 degrees. In a large bowl, toss the grated cheeses with the onions and season liberally with salt and pepper to mix well. Line the bottom of a 2qt casserole dish or baker with 1/3 of the cheese mixture. Add a layer of noodles. Alternate cheese and noodles again. Top with cheese and pour about 1/4 to 1/3 cup milk over the top. (The milk will run to the bottom of the dish – it shouldn’t be too liquidey.) Cover and bake for 30 minutes or until the top is slightly crispy.